1. Field of the Invention
The present invention relates to a rotor used for a small motor, and more particularly to a method for assembling a rotor in a DC motor with a brush, which has a 4-6 (four magnet poles and six salient poles of the armature core) structure.
2. Related Art
Referring to FIG. 8, there is illustrated a rotor 100 of a small motor with a brush which has a 2-3 (2 magnetic poles -3 salient poles) structure. In the rotor 100, an commutator unit 120 is provided with a armature core 101 and three commutator pieces 103 separated by slits 104. The armature core 101 has three salient poles 102 radially extended therefrom. Coils (not shown) of different phases are wound around the salient poles 102. The commutator unit 120 is tightly coupled to a shaft 107. In this type of the motor, an exactness of the timing of switching the current feeding to the coils of the phases depends largely on an accuracy of the alignment of the center line P1 of each salient pole 102 with the center line of the corresponding slit 104. Therefore, a misalignment of those center lines degrades the switching timing exactness, and causes cogging and an increase of torque ripple.
In a conventional measure taken for securing an exact alignment of the center lines, positioning protrusions 106 formed on an commutator holder 105 are fit into recesses 109 formed near a shaft hole 108 of the armature core 101.
In the DC motor of the 2-3 structure, the conventional measure secure secures an alignment accuracy to some degree. However, the following problem is inevitably created. The recesses 109 are excessively close to the center of the shaft hole 108. As seen from FIG. 9 showing the positioning protrusions 106 of the commutator holder 105 and the recesses 109 of the shaft hole 108, minute dimensional errors arising from dimensional inaccuracy of each part and of the part-to-part are enlarged in the radial direction. Therefore, it is impossible to expect the alignment accuracy as designed, in practical use.
In the DC motor of the 4-6 structure or higher grade structure (the number of salient poles is larger), the slits 104 and the commutator pieces 103 are increased in number. Therefore, an angle of each curved commutator piece 103 is halved, and as a result, the contact area of each commutator piece 103 with the commutator holder 105 is reduced. As a result, the commutator pieces 103 is easily tiltable, and more strict requirements are put on the assembling accuracy of the commutator unit 120 and position accuracy of the salient poles 102 of the armature core 101 to the commutator unit 120. In the DC motor of the 2-3 structure, a tolerable alignment (deviation angle) of the center line P1 of the salient pole 102 to the center line P2 of the slit 104 is 3.degree. or smaller. In the DC motor of the 4-6 structure, it is 1.5.degree. or smaller. Therefore, some adjustment is essential in manufacturing stage.
The terminals of the coils 110 wound on the salient poles 102 of the armature core 101 are connected to the armature risers 111, and soldering is applied thereto by a soldering iron. In the soldering process, pressing force and heat by and from the soldering iron possibly deform (denoted as H) of the risers 111 and the commutator holder 105, and degrade the roundness of the commutator pieces 103. When the DC motor of the 2-3 structure is compared with the DC motor of the 4-6 structure, a larger number of solderings must be applied to the risers 111 in the latter motor, and much heat stays there since the soldering points are more densely located. The thermal deformation H is more intensive.